Low diameter optical fiber

ABSTRACT

Small-radius coated optical fibers having large mode field diameter and low bending losses. The coated fiber may have an outer radius of 110 μm or less, while providing a mode field diameter of 9.0 μm or greater and a bending loss when wrapped about a 15 mm mandrel of 0.5 dB/km or less at wavelength of 1550 nm. The coated fiber may have a mode field diameter of 9.2 μm or greater and may have a bending loss at 1550 nm of 0.25 dB/km or less when wrapped about a 20 mm mandrel or a bending loss at 1550 nm of 0.02 dB/km or less when wrapped about a 30 mm mandrel.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/706,302, filed on May 7, 2015, which is a continuation of U.S. patentapplication Ser. No. 13/862,755, filed Apr. 15, 2013, the disclosures ofwhich are hereby incorporated by reference in their entirety herein.

FIELD OF INVENTION

The present disclosure relates generally to optical fibers. Moreparticularly, this disclosure relates to coated optical fibers having arefractive index profile with a depressed-index cladding region and athin, low modulus primary coating. Most particularly, this disclosurerelates to small-radius coated optical fibers that exhibit high modefield diameter and low bending losses.

TECHNICAL BACKGROUND

Coated optical fibers with small radii are attractive for reducing thesize of cables, decreasing cable cost, and efficiently using existingduct infrastructure for cable installations. Reduced-radii fiberstypically have the same glass radii as standard optical fibers (125 μm),but use thinner layers as primary and/or secondary coatings. Reducedcoating thickness, however, compromises the protective function of thecoatings. As a result, much effort in the field has been directed todeveloping new coating materials that maintain adequate protection atsmall thicknesses and new glass compositions or index profiles that cantolerate more pronounced bending without compromising signal intensityor quality. Although bend-insensitive coated optical fiber designs withreduced diameters have been proposed in the prior art (see, for example,US Patent Application Pub. No. 20100119202), the nominal mode fielddiameter (MFD) of these fibers at 1310 nm is typically only ˜8.6-8.8 μm.Such mode field diameters lead to high splicing/connectorization lossesupon connection of the reduced-diameter fiber to standard single modefibers (SMF) (which have a nominal MFD of about 9.2 μm).

To avoid signal losses when connecting low-diameter fibers to existing,standard single mode fibers, it would be desirable to develop an opticalfiber having a reduced radius and a mode field diameter that iscompatible with that of standard single mode fibers.

SUMMARY

The present disclosure provides coated optical fibers having a radius of110 μm or less, or 105 μm or less, or 100 μm or less, that possess largemode field diameters without experiencing significant bending-inducedsignal degradation. The reduced-radius coated fiber may comprise aninner glass region having a radius of at least 50 μm, or at least 55 μm,or at least 60 μm, or at least 62.5 μm in conjunction with surroundingprimary and secondary coatings. Representative fibers may include, inconcentric order, a glass core, a glass cladding, a primary coating anda secondary coating. The core may be a higher index glass region and maybe surrounded by a lower index cladding. The cladding may include one ormore inner cladding regions and an outer cladding region, where at leastone of the inner cladding regions may have a lower refractive index thanthe outer cladding region. The primary coating may be formed from alower modulus material and the secondary coating may be formed from ahigher modulus material.

The core may include silica glass or a silica-based glass. Silica-basedglass may be silica glass modified with an alkali metal (e.g. Na, K), analkaline earth metal (e.g. Mg, Ca), a column III element (e.g. B), or acolumn V element (e.g. P); or a dopant. The refractive index across thecore may be constant or variable. The core refractive index may be at amaximum at or near the center of the core and continuously decreases inthe direction of the outer core boundary. The core refractive indexprofile may be or may approximate a Gaussian profile, a super-Gaussianprofile, an α profile, or a step profile.

The cladding may include silica glass or a silica-based glass. Thesilica-based glass may be silica glass modified with an alkali metal(e.g. Na, K), an alkaline earth metal (e.g. Mg, Ca), a column IIIelement (e.g. B), or a column V element (e.g. P); or a dopant. Thecladding may include an inner cladding region and an outer claddingregion, where the inner cladding region may have a lower refractiveindex than the outer cladding region. The inner cladding region may havea constant or continuously varying refractive index. The inner claddingregion may have a refractive index that continuously decreases from itsinner boundary to its outer boundary. The continuous decrease may be alinear decrease. The refractive index of the inner cladding region mayform a trench in the index profile of the coated fiber. The index trenchmay be rectangular or triangular. The outer cladding region may have aconstant refractive index.

The cladding may include a first inner cladding region adjacent the coreand a second inner cladding region disposed between the first innercladding region and the outer cladding region. The refractive index ofthe second inner cladding region may be lower than the refractive indexof the first inner cladding region. The refractive index of the secondinner cladding region may be lower than the refractive index of theouter cladding region. The refractive index of the second inner claddingregion may be lower than the refractive indices of the first innercladding region and the outer cladding region.

The refractive index of the second inner cladding region may be constantor continuously varying. The second inner cladding region may have arefractive index that continuously decreases from its inner boundary toits outer boundary. The continuous decrease may be a linear decrease.The refractive index of the second inner cladding region may form atrench in the index profile of the coated fiber. The trench is a regionof depressed refractive index and may be rectangular or triangular. Theouter cladding region may have a constant refractive index.

The refractive index profiles of the core and cladding may be achievedthrough control of a spatial distribution of updopants and/ordowndopants in silica or silica-based glass.

The primary coating may be formed from a curable composition thatincludes an oligomer and a monomer. The oligomer may be a urethaneacrylate or a urethane acrylate with acrylate substitutions. Theurethane acrylate with acrylate substitutions may be a urethanemethacrylate. The oligomer may include urethane groups. The oligomer maybe a urethane acrylate that includes one or more urethane groups. Theoligomer may be a urethane acrylate with acrylate substitutions thatincludes one or more urethane groups. Urethane groups may be formed as areaction product of an isocyanate group and an alcohol group.

The primary coating may have an in situ modulus of elasticity of 1 MPaor less, or 0.50 MPa or less, or 0.25 MPa or less, or 0.20 MPa or less,or 0.19 MPa or less, or 0.18 MPa or less, or 0.17 MPa or less, or 0.16MPa or less, or 0.15 MPa or less. The glass transition temperature ofthe primary coating may be −15° C. or less, or −25° C. or less, or −30°C. or less, or −40° C. or less. The glass transition temperature of theprimary coating may be greater than −60° C., or greater than −50° C., orgreater than −40° C. The glass transition temperature of the primarycoating may be or between −60° C. and −15° C., or between −60° C. and−30° C., or between −60° C. and −40° C., or between −50° C. and −15° C.,or between −50° C. and −30° C., or between −50° C. and −40° C.

The secondary coating may be formed from a curable secondary compositionthat includes one or more monomers. The one or more monomers may includebisphenol-A diacrylate, or a substituted bisphenol-A diacrylate, or analkoxylated bisphenol-A diacrylate. The alkoxylated bisphenol-Adiacrylate may be an ethoxylated bisphenol-A diacrylate. The curablesecondary composition may further include an oligomer. The oligomer maybe a urethane acrylate or a urethane acrylate with acrylatesubstitutions. The secondary composition may be free of urethane groups,urethane acrylate compounds, urethane oligomers or urethane acrylateoligomers.

The secondary coating may be a material with a higher modulus ofelasticity and higher glass transition temperature than the primarycoating. The in situ modulus of elasticity of the secondary coating maybe 1200 MPa or greater, or 1500 MPa or greater, or 1800 MPa or greater,or 2100 MPa or greater, or 2400 MPa or greater, or 2700 MPa or greater.The secondary coating may have an in situ modulus between about 1500 MPaand 10,000 MPa, or between 1500 MPa and 5000 MPa. The in situ glasstransition temperature of the secondary coating may be at least 50° C.,or at least 55° C., or at least 60° C. or between 55° C. and 65° C.

The radius of the coated fibers coincides with the outer diameter of thesecondary coating. The radius of the coated fiber may be 110 μm or less,or 105 μm or less, or 100 μm or less. Within the coated fiber, the glassradius (coinciding with the outer diameter of the cladding) may be atleast 50 μm, or at least 55 μm, or at least 60 μm, or 62.5 μm. The glassmay be surrounded by the primary coating. The outer radius of theprimary coating may be 85 μm or less, 82.5 μm or less, or 80 μm or less,or 77.5 μm or less, or 75 μm or less. The balance of the coated fiberdiameter is provided by the secondary coating.

Coated fibers in accordance with the present disclosure may besmall-radius fibers that exhibit low bending losses while providing amode field diameter that minimizes losses associated with splicing andconnecting to standard single-mode fibers. The mode field diameter maybe 9.0 μm or greater, or 9.1 μm or greater, or 9.2 μm or greater at 1310nm.

The coated fibers may exhibit a bend loss at 1550 nm of less than 0.5dB/turn when wrapped around a mandrel with a 15 mm diameter, or lessthan 0.5 dB/turn when wrapped around a mandrel with a 20 mm diameter, orless than 0.25 dB/turn when wrapped around a mandrel with a 20 mmdiameter, or less than 0.02 dB/turn when wrapped around a mandrel with a30 mm diameter, or less than 0.012 dB/turn when wrapped around a mandrelwith a 30 mm diameter.

The optical and mechanical characteristics of the fibers of the presentdisclosure may be compliant with the G.652 standard. The fibers may havea cable cutoff wavelength of 1260 nm or less. The fibers may have a zerodispersion wavelength λ₀ in the range 1300 nm≤λ₀≤1324 nm.

Additional features and advantages will be set forth in the detaileddescription which follows, and in part will be readily apparent to thoseskilled in the art from the description or recognized by practicing theembodiments as described in the written description and claims hereof,as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary, and areintended to provide an overview or framework to understand the natureand character of the claims.

The accompanying drawings are included to provide a furtherunderstanding, and are incorporated in and constitute a part of thisspecification. The drawings illustrate one or more embodiment(s), andtogether with the description serve to explain principles and operationof the various embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction in cross-section of a fiber having acore, inner cladding region, outer cladding region, a primary coatingand a secondary coating.

FIG. 2 is a schematic depiction in cross-section of a fiber having acore, two inner cladding regions, an outer cladding region, a primarycoating and a secondary coating.

FIGS. 3A and 3B are schematic depictions of illustrative refractiveindex profiles.

FIG. 4 depicts a core-cladding refractive index profile having arectangular trench.

FIG. 5 depicts the core-cladding refractive index profile having atriangular trench.

FIG. 6 depicts the core-cladding refractive index profile having atriangular trench.

DETAILED DESCRIPTION

The present disclosure concerns coated optical fibers that may combine asmall diameter, a large mode field diameter, and low microbending loss.A brief explanation of selected terminology used herein is nowpresented:

The “refractive index profile” is the relationship between refractiveindex or relative refractive index and fiber radius.

The “relative refractive index percent” is defined as

${\Delta\;\%} = {100\frac{{n^{2}(r)} - n_{s}^{2}}{2\;{n^{2}(r)}}}$where n(r) is the refractive index of the fiber at the radial distance rfrom the fiber's centerline, unless otherwise specified, and n_(s) isthe refractive index of pure silica at a wavelength of 1550 nm. As usedherein, the relative refractive index is represented by Δ (or “delta”),Δ % (or “delta %”), or %, all of which are used interchangeably herein,and its values are given in units of percent or %, unless otherwisespecified. Relative refractive index may also be expressed as Δ(r) orΔ(r) %.

“Chromatic dispersion”, which may also be referred to as “dispersion”,of a waveguide fiber is the sum of the material dispersion, thewaveguide dispersion, and the intermodal dispersion at a wavelength λ.In the case of single-mode waveguide fibers, the inter-modal dispersionis zero. Dispersion values in a two-mode regime assume intermodaldispersion is zero. The zero dispersion wavelength (λ₀) is thewavelength at which the dispersion has a value of zero. Dispersion slopeis the rate of change of dispersion with respect to wavelength.

The term “α-profile” refers to a relative refractive index profile Δ(r)that has the following functional form:

${\Delta(r)} = {{\Delta( r_{0} )}\lbrack {1 - \lbrack \frac{{r - r_{0}}}{( {r_{1} - r_{0}} )} \rbrack^{\alpha}} \rbrack}$where r_(o) is the point at which Δ(r) is maximum, r₁ is the point atwhich Δ(r) is zero, and r is in the range r_(i)≤r≤r_(f), where r_(i) isthe initial point of the α-profile, r_(f) is the final point of theα-profile, and α is a real number.

The mode field diameter (MFD) is measured using the Petermann II methodand was determined from:

MFD = 2w$w^{2} = {2\frac{\int_{0}^{\infty}{( {f(r)} )^{2}{rdr}}}{\int_{0}^{\infty}{( \frac{{df}(r)}{dr} )^{2}{rdr}}}}$where f(r) is the transverse electric field distribution of the guidedlight and r is radial position in the fiber.

The bend resistance of a waveguide fiber may be gauged by inducedattenuation under prescribed test conditions. Various tests are used toassess bending losses including the lateral load microbend test, pinarray test, and mandrel wrap test.

In the lateral load test, a prescribed length of waveguide fiber isplaced between two flat plates. A #70 wire mesh is attached to one ofthe plates. A known length of waveguide fiber is sandwiched between theplates and a reference attenuation at a selected wavelength (typicallywithin the range of 1200-1700 nm, e.g., 1310 nm or 1550 nm or 1625 nm)is measured while the plates are pressed together with a force of 30Newtons. A 70 Newton force is then applied to the plates and theincrease in attenuation at the selected wavelength in dB/m is measured.The increase in attenuation is the lateral load wire mesh (LLWM)attenuation of the waveguide.

The macrobend resistance of the fiber can be gauged by measuring theinduced attenuation increase in a mandrel wrap test. In the mandrel wraptest, the fiber is wrapped one or more times around a cylindricalmandrel having a specified diameter and the increase in attenuation at aspecified wavelength due to the bending is determined. Attenuation inthe mandrel wrap test is expressed in units of dB/turn, where one turnrefers to one revolution of the fiber about the mandrel.

The “pin array” bend test is used to compare the relative resistance ofwaveguide fiber to bending. To perform this test, attenuation loss at aselected wavelength is measured for a waveguide fiber in a configurationwith essentially no bending loss. The waveguide fiber is then wovenabout the pin array and the attenuation at the selected wavelength(typically within the range of 1200-1700 nm, e.g., 1310 nm or 1550 nm or1625 nm) is again measured. The loss induced by bending is thedifference between the two measured attenuations. The pin array is a setof ten cylindrical pins arranged in a single row and held in a fixedvertical position on a flat surface. The pin spacing is 5 mm, center tocenter. The pin diameter is 0.67 mm. During testing, sufficient tensionis applied to make the waveguide fiber conform to a portion of the pinsurface.

The fiber cutoff can be measured by the standard 2 m fiber cutoff test,FOTP-80 (EIA-TIA-455-80), to yield the “fiber cutoff wavelength”, alsoknown as the “2 m fiber cutoff” or “measured cutoff”. The FOTP-80standard test is performed to either strip out the higher order modesusing a controlled amount of bending, or to normalize the spectralresponse of the fiber to that of a multimode fiber.

The cabled cutoff wavelength, or “cable cutoff” (also known as the“22-meter cutoff”) is typically lower than the measured fiber cutoff dueto higher levels of bending and mechanical pressure in the cableenvironment. The actual cabled condition can be approximated by thecabled cutoff test described in the EIA-445 Fiber Optic Test Procedures,which are part of the EIA-TIA Fiber Optics Standards (ElectronicsIndustry Alliance-Telecommunications Industry Association Fiber OpticsStandards, more commonly known as FOTP's). Cable cutoff measurement isdescribed in EIA-455-170 Cable Cutoff Wavelength of Single-mode Fiber byTransmitted Power, or “FOTP-170”. Unless otherwise noted herein, opticalproperties (such as dispersion, dispersion slope, etc.) are reported forthe LP01 mode.

The present disclosure provides small-radius coated fibers withexcellent microbending and macrobending performance and a mode fielddiameter that may permit splicing and connecting to standard single-modefibers with minimal losses. The coated fibers of the present disclosuremay overcome sacrifices in mode field diameter and/or bending lossesthat have accompanied efforts in the prior art to achieve small-radiusfibers. With the present coated fibers, small radii may be achievablewithout sacrificing mode field diameter or bending performance. Thepresent disclosure accordingly may provide compact coated fibers thatcan be assembled in high density configurations for internalinstallations and yet provide good matching and low losses whenintegrated with external single-mode fibers. Different profile designsare outlined below that result in good fiber microbend and macrobendperformance even when the thickness of the coating layers is small.Mechanical properties, compositions, and geometry of reduced-thicknessprimary and secondary coating layers that may yield low microbending andmacrobending losses and good puncture resistance are disclosed. Unlessotherwise specified, all wavelength-dependent results are based on awavelength of 1550 nm.

The present coated fibers may include a cladding having two regions anda refractive index profile that differs in the two regions. The designof the refractive index profile of the cladding may diminish thesensitivity of the coated fiber to bending and may enable use of aprimary coating with reduced thickness relative to prior art coatedfibers. A thinner primary coating leads to a reduction in overall coatedfiber diameter to provide compact coated fibers that can be denselypacked and/or readily installed in existing fiber infrastructure. Themechanical properties of the primary coating may be chosen such thatgood microbending performance of the coated fiber is achieved, even whenthe thickness of the primary coating is reduced.

The coated fibers of the present disclosure may include a core,cladding, primary coating, and secondary coating, where the cladding mayinclude two or more regions with differing refractive index profiles. Aschematic cross-sectional depiction of a first of many coated fibers inaccordance with the present disclosure is shown in FIG. 1. Fiber 10includes core 20, cladding 30, primary coating 40, and secondary coating50. Cladding 30 includes inner cladding region 33 and outer claddingregion 37. The schematic cross-section of a second of many coated fibersin accordance with the present disclosure is shown in FIG. 2. Fiber 60includes core 70, cladding 80, primary coating 90 and secondary coating100. Cladding 80 includes first inner cladding region 81, second innercladding region 83, and outer cladding region 85.

The core and cladding may be silica or silica-based glass and mayoptionally include an updopant or a downdopant. Silica-based glass maybe silica glass modified by an alkali or alkaline earth element, or acolumn III element (e.g. B, Al), or a column V element (e.g. P). Theradius of the core may be in the range of 4-10 μm for single-mode fiber.The cladding may include two or more regions that differ in refractiveindex profile and may extend to an outer radius of at least 50 μm, or atleast 55 μm, or at least 60 μm, or 62.5 μm.

The refractive index across the core may be constant or variable. Thecore refractive index may be at a maximum at or near the center of thecore and may continuously decrease in the direction of the outer coreboundary. The core refractive index profile may be or may approximate aGaussian profile, a super-Gaussian profile, an α-profile, or a stepprofile.

The cladding may include an inner cladding region and an outer claddingregion, where the inner cladding region may have a lower refractiveindex than the outer cladding region. The refractive index of the innercladding region may be constant or continuously varying. The innercladding region may have a refractive index that continuously decreasesfrom its inner boundary to its outer boundary. The continuous decreasemay be a linear decrease. The refractive index of the inner claddingregion may form a trench in the refractive index profile of the coatedfiber. The trench is a depressed index region and may be rectangular ortriangular. The outer cladding region may have a constant orcontinuously varying refractive index. The minimum refractive index ofthe inner core region may be less than the maximum refractive index ofthe outer cladding region.

The cladding may include a first inner cladding region adjacent the coreand a second inner cladding region disposed between the first innercladding region and the outer cladding region. The refractive index ofthe second inner cladding region may be lower than the refractive indexof the first inner cladding region. The minimum refractive index of thesecond inner cladding region may be lower than the maximum refractiveindex of the first inner cladding region. The refractive index of thesecond inner cladding region may be lower than the refractive index ofthe outer cladding region. The minimum refractive index of the secondinner cladding region may be lower than the maximum refractive index ofthe outer cladding region. The refractive index of the second innercladding region may be lower than the refractive indices of the firstinner cladding region and the outer cladding region. The minimumrefractive index of the second inner cladding region may be lower thanthe maximum refractive indices of the first inner cladding region andthe outer cladding region.

The refractive index of the second inner cladding region may be constantor continuously varying. The second inner cladding region may have arefractive index that continuously decreases from its inner boundary toits outer boundary. The continuous decrease may be a linear decrease.The refractive index of the second inner cladding region may form atrench in the refractive index profile of the coated fiber. The trenchis a depressed index region and may be rectangular or triangular. Thedepressed index region may characterized by a profile moat volume, V₃,in units of % μm², equal to:V ₃=2∫_(r2) ^(r3)Δ(r)rdrThe magnitude |V₃| of the moat volume may be at least 30% μm², or atleast 50% μm², or at least 65% μm². The magnitude |V₃| of the moatvolume may also be less than 80% μm², or less than 75% μm², or between30% μm² and 80% μm², inclusive. The terms “moat” and “trench” are usedinterchangeably herein.

Representative refractive index profiles for the core and cladding arepresented in FIGS. 3A and 3B. FIG. 3A shows a rectangular trench profilefor a fiber (101) having a core (1) with outer radius r₁ and refractiveindex Δ₁, a first inner cladding region (2) extending from radialposition r₁ to radial position r₂ and having refractive index Δ₂, asecond inner cladding region (3) extending from radial position r₂ toradial position r₃ and having refractive index Δ₃, and an outer claddingregion (4) extending from radial position r₃ to radial position r₄ andhaving refractive index Δ₄. In the profile of FIG. 3A, the second innercladding region (3) may be referred to herein as a rectangular trenchand may have a constant refractive index that is less than therefractive indices of the first inner cladding region (2) and the outercladding region (4). The core (1) may have the highest refractive indexin the profile. The core (1) may include a lower index region at or nearthe centerline (known in the art as a “centerline dip”). It should benoted that the first inner cladding region (2) is optional and may beeliminated.

FIG. 3B shows a triangular trench refractive index profile for a fiber(101) having a core (1) with radius r₁ and refractive index Δ₁ with amaximum A_(1MAX), a first inner cladding region (2) extending fromradial position r₁ to radial position r₂ and having refractive index Δ₂,a second inner cladding region (3) extending from radial position r₂ toradial position r₃ and having refractive index Δ₃ with a minimumΔ_(3MIN), and an outer cladding region (4) extending from radialposition r₃ to radial position r₄ and having refractive index Δ₄. In theprofile of FIG. 3B, the second inner cladding region (3) may be referredto herein as a triangular trench and may have a continuously decreasingrefractive index between radial positions r₂ and r₃, where the averageand minimum refractive index of the triangular trench may be less thanthe refractive indices of the first inner cladding region (2) and theouter cladding region (4). The core (1) may have the highest refractiveindex in the profile. The core (1) may include a lower index region ator near the centerline with a refractive index less than A_(1MAX). Itshould be noted that the first inner cladding region (2) is optional andmay be eliminated.

The refractive index profiles of the core and cladding may be achievedthrough control of the spatial distribution of dopants or modifiers insilica or silica-based glass. Updopants (e.g. GeO₂, Al₂O₃, P₂O₅, TiO₂,Cl, Br) may be used to create regions of increased refractive index anddowndopants (e.g. F, B₂O₃, non-periodic voids) may be used to createregions of decreased refractive index. Regions of constant refractiveindex may be formed by not doping or by doping at a uniformconcentration. Regions of variable refractive index may be formedthrough non-uniform spatial distributions of dopants. The triangulartrench shown in FIG. 3B, for example, may be established byincorporating F as a downdopant with a non-uniform spatial concentrationprofile. The concentration of F at radial position r₂ may be less thanthe concentration of F at radial position r₃.

The coated fiber may include regions interposed between the core andfirst inner cladding region, or between the first inner cladding regionand the second inner cladding region, or between the second innercladding region and the outer cladding region, or between the outercladding region and the primary coating, or between the primary coatingand the secondary coating. The fiber may have a core with an outerradius r₁ and refractive index Δ₁ with a maximum value Δ_(1MAX) and aminimum value Δ_(1MIN), a first inner cladding region having an outerradius r₂ and having refractive index Δ₂ with a maximum value Δ_(2MAX)and a minimum value Δ_(2MIN), a second inner cladding region having anouter radius r₃ and having refractive index Δ₃ with a maximum valueΔ_(3MAX) and a minimum value Δ_(3MIN), an outer cladding region havingan outer radius r₄ and having refractive index Δ₄ with a maximum valueΔ_(4MAX) and a minimum value Δ_(4MIN), a primary coating having outerradius r₅, and a secondary coating having outer radius r₆, wherer₆>r₅>r₄>r₃>r₂>r₁.

The core and cladding of the present coated fibers may be produced in asingle-step operation or multi-step operation by methods which are wellknown in the art. Suitable methods include: the double crucible method,rod-in-tube procedures, and doped deposited silica processes, alsocommonly referred to as chemical vapor deposition (“CVD”) or vapor phaseoxidation. A variety of CVD processes are known and are suitable forproducing the core and cladding layer used in the coated optical fibersof the present invention. They include external CVD processes, axialvapor deposition processes, modified CVD (MCVD), inside vapordeposition, and plasma-enhanced CVD (PECVD).

The glass portion of the coated fibers may be drawn from a speciallyprepared, cylindrical preform which has been locally and symmetricallyheated to a temperature sufficient to soften the glass, e.g., atemperature of about 2000° C. for a silica glass. As the preform isheated, such as by feeding the preform into and through a furnace, aglass fiber is drawn from the molten material. See, for example, U.S.Pat. Nos. 7,565,820; 5,410,567; 7,832,675; and 6,027,062; thedisclosures of which are hereby incorporated by reference herein, forfurther details about fiber making processes.

The primary coating may have a lower modulus than the secondary coating.The primary coating may be formed from a primary composition thatincludes a curable oligomer. The curable primary composition may alsoinclude monomers, a polymerization initiator, and one or more additives.Unless otherwise specified or implied herein, the weight percent (wt %)of a particular component in a curable primary composition refers to theamount of the component present in the curable primary composition on anadditive-free basis. Generally, the weight percents of the monomer(s),oligomer(s), and initiator(s) sum to 100%. When present, the amount ofan additive is reported herein in units of parts per hundred (pph)relative to the combined amounts of monomer(s), oligomer(s), andinitiator(s). An additive present at the 1 pph level, for example, ispresent in an amount of 1 g for every 100 g of combined monomer(s),oligomer(s), and initiator(s).

The oligomer of the curable primary composition may be a urethaneacrylate oligomer, or a urethane acrylate oligomer that includes one ormore urethane groups, or a urethane acrylate oligomer that includes oneor more aliphatic urethane groups, or a urethane acrylate oligomer thatincludes a single urethane group, or a urethane acrylate oligomer thatincludes a single aliphatic urethane group. The urethane group may beformed from a reaction between an isocyanate group and an alcohol group.

The oligomer may be an acrylate-terminated oligomer. Preferredacrylate-terminated oligomers for use in the primary curablecompositions include BR3731, BR3741, BR582 and KWS4131, from DymaxOligomers & Coatings; polyether urethane acrylate oligomers (e.g.,CN986, available from Sartomer Company); polyester urethane acrylateoligomers (e.g., CN966 and CN973, available from Sartomer Company, andBR7432, available from Dymax Oligomers & Coatings); polyether acrylateoligomers (e.g., GENOMER 3456, available from Rahn AG); and polyesteracrylate oligomers (e.g., EBECRYL 80, 584 and 657, available from CytecIndustries Inc.). Other oligomers are described in U.S. Pat. Nos.4,609,718; 4,629,287; and 4,798,852, the disclosures of which are herebyincorporated by reference in their entirety herein.

The oligomer of the primary curable composition may include a soft blockwith a number average molecular weight (M_(n)) of about 4000 g/mol orgreater. Examples of such oligomers are described in U.S. patentapplication Ser. No. 09/916,536, the disclosure of which is incorporatedby reference herein in its entirety. The oligomers may have flexiblebackbones, low polydispersities, and/or may provide cured coatings oflow crosslink densities.

The oligomers may be used singly, or in combination to control coatingproperties. The total oligomer content of the primary curablecomposition may be between about 5 wt % and about 95 wt %, or betweenabout 25 wt % and about 65 wt %, or between about 35 wt % and about 55wt %.

The monomer component of the primary curable composition may be selectedto be compatible with the oligomer, to provide a low viscosityformulation, and/or to increase the refractive index of the primarycoating. The monomer may also be selected to provide curablecompositions having decreased gel times and low moduli. The primarycurable composition may include a single monomer or a combination ofmonomers. The monomers may include ethylenically-unsaturated compounds,ethoxylated acrylates, ethoxylated alkylphenol monoacrylates, propyleneoxide acrylates, n-propylene oxide acrylates, isopropylene oxideacrylates, monofunctional acrylates, monofunctional aliphatic epoxyacrylates, multifunctional acrylates, multifunctional aliphatic epoxyacrylates, and combinations thereof. The monomer component may includecompounds having the general formula R₂—R₁—O—(CH₂CH₃CH—O)_(n)—COCH═CH₂,where R₁ and R₂ are aliphatic, aromatic, or a mixture of both, and n=1to 10, or R₁—O—(CH₂CH₃CH—O)_(n)—COCH═CH₂, where R₁ is aliphatic oraromatic, and n=1 to 10. Representative examples include ethylenicallyunsaturated monomers such as lauryl acrylate (e.g., SR335 available fromSartomer Company, Inc., AGEFLEX FA12 available from BASF, and PHOTOMER4812 available from IGM Resins), ethoxylated nonylphenol acrylate (e.g.,SR504 available from Sartomer Company, Inc. and PHOTOMER 4066 availablefrom IGM Resins), caprolactone acrylate (e.g., SR495 available fromSartomer Company, Inc., and TONE M-100 available from Dow Chemical),phenoxyethyl acrylate (e.g., SR339 available from Sartomer Company,Inc., AGEFLEX PEA available from BASF, and PHOTOMER 4035 available fromIGM Resins), isooctyl acrylate (e.g., SR440 available from SartomerCompany, Inc. and AGEFLEX FA8 available from BASF), tridecyl acrylate(e.g., SR489 available from Sartomer Company, Inc.), isobornyl acrylate(e.g., SR506 available from Sartomer Company, Inc. and AGEFLEX IBOAavailable from CPS Chemical Co.), tetrahydrofurfuryl acrylate (e.g.,SR285 available from Sartomer Company, Inc.), stearyl acrylate (e.g.,SR257 available from Sartomer Company, Inc.), isodecyl acrylate (e.g.,SR395 available from Sartomer Company, Inc. and AGEFLEX FA10 availablefrom BASF), 2-(2-ethoxyethoxy)ethyl acrylate (e.g., SR256 available fromSartomer Company, Inc.), epoxy acrylate (e.g., CN120, available fromSartomer Company, and EBECRYL 3201 and 3604, available from CytecIndustries Inc.), lauryloxyglycidyl acrylate (e.g., CN130 available fromSartomer Company) and phenoxyglycidyl acrylate (e.g., CN131 availablefrom Sartomer Company) and combinations thereof.

The monomer component of the primary curable composition may alsoinclude a multifunctional (meth)acrylate. As used herein, the term“(meth)acrylate” means acrylate or methacrylate. Multifunctional(meth)acrylates are (meth)acrylates having two or more polymerizable(meth)acrylate moieties per molecule. The multifunctional (meth)acrylatemay have three or more polymerizable (meth)acrylate moieties permolecule. Examples of multifunctional (meth)acrylates includedipentaerythritol monohydroxy pentaacrylate (e.g., PHOTOMER 4399available from IGM Resins); methylolpropane polyacrylates with andwithout alkoxylation such as trimethylolpropane triacrylate,ditrimethylolpropane tetraacrylate (e.g., PHOTOMER 4355, IGM Resins);alkoxylated glyceryl triacrylates such as propoxylated glyceryltriacrylate with propoxylation being 3 or greater (e.g., PHOTOMER 4096,IGM Resins); and erythritol polyacrylates with and without alkoxylation,such as pentaerythritol tetraacrylate (e.g., SR295, available fromSartomer Company, Inc. (Westchester, Pa.)), ethoxylated pentaerythritoltetraacrylate (e.g., SR494, Sartomer Company, Inc.), dipentaerythritolpentaacrylate (e.g., PHOTOMER 4399, IGM Resins, and SR399, SartomerCompany, Inc.), tripropyleneglycol di(meth)acrylate, propoxylatedhexanediol di(meth)acrylate, tetrapropyleneglycol di(meth)acrylate,pentapropyleneglycol di(meth)acrylate. A multifunctional (meth)acrylatemay be present in the primary curable composition at a concentration offrom 0.05-15 wt %, or from 0.1-10 wt %.

The monomer component of the primary curable compositions may include anN-vinyl amide such as an N-vinyl lactam, or N-vinyl pyrrolidinone, orN-vinyl caprolactam. The N-vinyl amide monomer may be present in theprimary curable composition at a concentration from 0.1-40 wt %, or from2-10 wt %.

The curable primary coating composition may include one or moremonofunctional (meth)acrylate monomers in an amount from 5-95 wt %, orfrom 30-75 wt %, or from 40-65 wt %. The curable primary coatingcomposition may include one or more monofunctional aliphatic epoxyacrylate monomers in an amount from 5-40 wt %, or from 10-30 wt %.

The monomer component of the primary curable composition may include ahydroxyfunctional monomer. A hydroxyfunctional monomer is a monomer thathas a pendant hydroxy moiety in addition to other reactive functionalitysuch as (meth)acrylate. Examples of hydroxyfunctional monomers includingpendant hydroxyl groups include caprolactone acrylate (available fromDow Chemical as TONE M-100); poly(alkylene glycol) mono(meth)acrylates,such as poly(ethylene glycol) monoacrylate, poly(propylene glycol)monoacrylate, and poly(tetramethylene glycol) monoacrylate (eachavailable from Monomer, Polymer & Dajac Labs); 2-hydroxyethyl(meth)acrylate, 3-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl(meth)acrylate (each available from Aldrich).

The hydroxyfunctional monomer may be present in an amount sufficient toimprove adhesion of the primary coating to the optical fiber. Thehydroxyfunctional monomer may be present in the primary curablecomposition in an amount between about 0.1 wt % and about 25 wt %, or inan amount between about 5 wt % and about 8 wt %. The use of thehydroxyfunctional monomer may decrease the amount of adhesion promoternecessary for adequate adhesion of the primary coating to the opticalfiber. The use of the hydroxyfunctional monomer may also tend toincrease the hydrophilicity of the primary coating. Hydroxyfunctionalmonomers are described in more detail in U.S. Pat. No. 6,563,996, thedisclosure of which is hereby incorporated by reference in its entirety.

The total monomer content of the primary curable composition may bebetween about 5 wt % and about 95 wt %, or between about 30 wt % andabout 75 wt %, or between about 40 wt % and about 65 wt %.

The monomer present in the primary curable composition may include anN-vinyl amide monomer at a concentration of 0.1 to 40 wt % or 2 to 10 wt% in combination with one or more difunctional urethane acrylateoligomers in an amount from 5 to 95 wt %, or from 25 to 65 wt % or from35 to 55 wt %.

The primary coating composition may include one or more monofunctional(meth)acrylate monomers in an amount of from about 5 to 95 wt %; anN-vinyl amide monomer in an amount of from about 0.1 to 40 wt %; and oneor more difunctional urethane acrylate oligomers that include a polyolreacted with an isocyanate to form a urethane, where the oligomer ispresent in an amount of from about 5 to 95 wt %. The polyol may be apolypropylene glycol and the isocyanate may be an aliphaticdiisocyanate.

The primary coating composition may include one or more monofunctional(meth)acrylate monomers in an amount of from about 40 to 65% by weight;an N-vinyl amide monomer in an amount of from about 2 to 10% by weight;and one or more polypropylene glycol-based difunctional urethaneacrylate oligomers in an amount of from about 35 to 60% by weight.

The glass transition temperature of the primary coating may influencethe microbend performance of the fibers at low temperature. It may bedesirable for the primary coating to have a glass transition temperaturebelow the lowest projected use temperature of the coated optical fiber.The glass transition temperature of the primary coating may be −15° C.or less, or −25° C. or less, or −30° C. or less, or −40° C. or less. Theglass transition temperature of the primary coating may be greater than−60° C., or greater than −50° C., or greater than −40° C. The glasstransition temperature of the primary coating may be or between −60° C.and −15° C., or between −60° C. and −30° C., or between −60° C. and −40°C., or between −50° C. and −15° C., or between −50° C. and −30° C., orbetween −50° C. and −40° C.

The primary coating may have a lower modulus of elasticity than thesecondary coating. A low modulus may allow the primary coating toprotect the core and cladding by efficiently dissipating internalstresses that arise when the exterior of the fiber is bent or subjectedto an external force. As used herein, in situ modulus of the primarycoating is the modulus measured by the technique that is now described.

A six-inch fiber sample is used for the measurement of the in situmodulus of the primary coating. A one-inch section from the center ofthe six-inch sample is window stripped and wiped with isopropyl alcohol.The sample is mounted on a sample holder/alignment stage equipped with10 mm×5 mm aluminum tabs to which the sample is glued. The two tabs areset so that the 10 mm length is laid horizontally with a 5 mm gapbetween two tabs. The fiber is laid horizontally on the sample holderacross the tabs. The coated end of the fiber is positioned on one taband extended halfway into the 5 mm space between the tabs and thestripped glass is positioned over the other half of the 5 mm gap and onthe other tab. The sample is lined up and then moved out of the way sothat a small dot of glue can be applied to the half of each tab closestto the 5 mm gap. The fiber is then brought back over the tabs andcentered. The alignment stage is then raised until the glue just touchesthe fiber. The coated end is then pulled through the glue such that themajority of the sample in the 5 mm gap between the tabs is strippedglass. The very tip of the coated end is left extended beyond the glueon the tab so that the region to be measured is left exposed. The sampleis left to dry. The length of fiber fixed to the tabs is trimmed to 5mm. The coated length embedded in glue, the non-embedded length (betweenthe tabs), and the end-face primary diameter are measured.

Measurements can be performed on an instrument such as a RheometricsDMTA IV at a constant strain of 9e-6 l/s for a time of forty-fiveminutes at room temperature (˜21° C.). The gauge length is 15 mm. Forceand the change in length are recorded and used for the calculation ofprimary modulus. Samples are prepared by removing any epoxy from thetabs that would interfere with the 15 mm clamping length to insure thereis no contact with the fiber and that the sample is secured squarely tothe clamps. Once the instrument force is zeroed out, the non-coated endis mounted to the lower clamp (measurement probe). The tab containingthe coated end of the fiber is mounted to the upper (fixed) clamp. Thetest is then executed and the sample is removed once the analysis iscomplete.

The in situ modulus of the primary coating may be 1 MPa or less, or 0.50MPa or less, or 0.25 MPa or less, or 0.20 MPa or less, or 0.19 MPa orless, or 0.18 MPa or less, or 0.17 MPa or less, or 0.16 MPa or less, or0.15 MPa or less, or between 0.01 MPa and 1.0 MPa, or between 0.01 MPaand 0.50 MPa, or between 0.01 MPa and 0.20 MPa.

The primary curable composition may also include polymerizationinitiators, antioxidants, and other additives familiar to the skilledartisan.

The polymerization initiator may facilitate initiation of thepolymerization process associated with the curing of the primarycomposition to form the primary coating. Polymerization initiators mayinclude thermal initiators, chemical initiators, electron beaminitiators, and photoinitiators. For many (meth)acrylate-based coatingformulations, photoinitiators such as ketonic photoinitiating additivesand/or phosphine oxide additives may be employed. When used in thephotoformation of the primary coating of the present disclosure, thephotoinitiator may be present in an amount sufficient to provide rapidultraviolet curing.

Suitable photoinitiators may include 1-hydroxycyclohexylphenyl ketone(e.g., IRGACURE 184 available from BASF));bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide (e.g.,commercial blends IRGACURE 1800, 1850, and 1700 available from BASF);2,2-dimethoxy-2-phenylacetophenone (e.g., IRGACURE 651, available fromBASF); bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide (IRGACURE 819);(2,4,6-trimethylbenzoyl)diphenyl phosphine oxide (LUCIRIN TPO, availablefrom BASF (Munich, Germany));ethoxy(2,4,6-trimethylbenzoyl)-phenylphosphine oxide (LUCIRIN TPO-L fromBASF); and combinations thereof.

The photoinitiator component of the primary curable composition mayconsist of a single photoinitiator or a combination of two or morephotoinitiators. The total photoinitiator content of the primary curablecomposition may be up to about 10 wt %, or between about 0.5 wt % andabout 6 wt %.

In addition to monomer(s), oligomer(s), and polymerization initiator(s),the primary curable composition may include other additives such as anadhesion promoter, a strength additive, a reactive diluent, anantioxidant, a catalyst, a stabilizer, an optical brightener, aproperty-enhancing additive, an amine synergist, a wax, a lubricant,and/or a slip agent. Some additives may operate to control thepolymerization process, thereby affecting the physical properties (e.g.,modulus, glass transition temperature) of the polymerization productformed from the primary curable composition. Other additives may affectthe integrity of the polymerization product of the primary curablecomposition (e.g., protect against de-polymerization or oxidativedegradation). For example, the primary curable composition may include acarrier, as described in U.S. Pat. Nos. 6,326,416 and 6,539,152, thedisclosures of which are hereby incorporated by reference herein.

It may be desirable to include an adhesion promoter in the primarycurable composition. An adhesion promoter is a compound that mayfacilitate adhesion of the primary coating and/or primary composition tothe cladding. Suitable adhesion promoters include alkoxysilanes,organotitanates, and zirconates. Representative adhesion promotersinclude 3-mercaptopropyl-trialkoxysilane (e.g., 3-MPTMS, available fromGelest (Tullytown, Pa.)); bis(trialkoxysilyl-ethyl)benzene;acryloxypropyltrialkoxysilane (e.g.,(3-acryloxypropyl)-trimethoxysilane, available from Gelest),methacryloxypropyltrialkoxysilane, vinyltrialkoxysilane,bis(trialkoxysilylethyl)hexane, allyltrialkoxysilane,styrylethyltrialkoxysilane, and bis(trimethoxysilylethyl)benzene(available from United Chemical Technologies (Bristol, Pa.)); see U.S.Pat. No. 6,316,516, the disclosure of which is hereby incorporated byreference in its entirety herein.

The adhesion promoter may be present in the primary composition in anamount between about 0.02 pph to about 10 pph, or between about 0.05 pphand 4 pph, or between about 0.1 pph to about 2 pph, or between about 0.1pph to about 1 pph.

The primary coating composition may also include a strength additive, asdescribed in U.S. Published Patent Application No. 20030077059, thedisclosure of which is hereby incorporated by reference herein in itsentirety. Representative strength additives include mercapto-functionalcompounds, such as N-(tert-butoxycarbonyl)-L-cysteine methyl ester,pentaerythritol tetrakis(3-mercaptopropionate),(3-mercaptopropyl)-trimethoxysilane; (3-mercaptopropyl)trimethoxysilane,and dodecyl mercaptan. The strength additive may be present in theprimary curable composition in an amount less than about 1 pph, or in anamount less than about 0.5 pph, or in an amount between about 0.01 pphand about 0.1 pph.

A representative antioxidant is thiodiethylenebis[3-(3,5-di-tert-butyl)-4-hydroxyphenyl) propionate] (e.g., IRGANOX1035, available from BASF).

It may be desirable to include an optical brightener in the primarycurable composition. Representative optical brighteners include TINOPALOB (available from BASF); Blankophor KLA (available from Bayer);bisbenzoxazole compounds; phenylcoumarin compounds; andbis(styryl)biphenyl compounds. The optical brightener may be present inthe primary curable composition at a concentration of 0.005 pph-0.3 pph.

It may also be desirable to include an amine synergist in the primarycurable composition. Representative amine synergists includetriethanolamine; 1,4-diazabicyclo[2.2.2]octane (DABCO), triethylamine,and methyldiethanolamine. The amine synergist may be present at aconcentration of 0.02 pph-0.5 pph.

The secondary coating may protect the fiber from mechanical damage andthe external environment. The secondary coating may be formed from acurable secondary composition that includes one or more monomers. Themonomers may include ethylenically unsaturated compounds. The curablesecondary composition may also include one or more oligomers, one ormore polymerization initiators, and one or more additives as describedmore fully herein.

Unless otherwise specified or implied herein, the weight percent (wt %)of a particular component in a curable secondary composition refers tothe amount of the component present in the curable secondary compositionon an additive-free basis. Generally, the weight percents of themonomer(s), oligomer(s), and initiator(s) sum to 100%. When present, theamount of an additive is reported herein in units of parts per hundred(pph) relative to the combined amounts of monomer(s), oligomer(s), andinitiator(s). An additive present at the 1 pph level, for example, ispresent in an amount of 1 g for every 100 g of combined monomer(s),oligomer(s), and initiator(s).

In order to reduce cost, the oligomer content urethane oligomer content,or urethane acrylate oligomer content of the secondary composition maybe minimized. Relative to the prevailing secondary compositions known inthe art, the oligomer content, urethane oligomer content, or urethaneacrylate oligomer content of the present secondary composition isparticularly low. Oligomers, urethane oligomers, or urethane acrylateoligomers may be present as a minority component or completely absentfrom the secondary composition of the present disclosure. Oligomers,urethane oligomers, or urethane acrylate oligomers may be present in thesecondary composition in an amount of about 3 wt % or less, or in anamount of about 2 wt % or less, or in an amount of about 1 wt % or less.The secondary composition may also be devoid of oligomers, urethaneoligomers, or urethane acrylate oligomers.

The monomer component of the curable secondary composition may includeone or more monomers. The one or more monomers may be present in thesecondary composition in an amount of 50 wt % or greater, or in anamount from about 75 wt % to about 99 wt %, or in an amount from about80 wt % to about 99 wt % or in an amount from about 85 wt % to about 98wt %.

The monomer component of the curable secondary composition may includeethylenically unsaturated compounds. The ethylenically unsaturatedmonomers may be monofunctional or polyfunctional. The functional groupsmay be polymerizable groups and/or groups that facilitate or enablecrosslinking. In combinations of two or more monomers, the constituentmonomers may be monofunctional, polyfunctional, or a combination ofmonofunctional and polyfunctional compounds. Suitable functional groupsfor ethylenically unsaturated monomers include, without limitation,(meth)acrylates, acrylamides, N-vinyl amides, styrenes, vinyl ethers,vinyl esters, acid esters, and combinations thereof.

Exemplary monofunctional ethylenically unsaturated monomers include,without limitation, hydroxyalkyl acrylates such as2-hydroxyethyl-acrylate, 2-hydroxypropyl-acrylate, and2-hydroxybutyl-acrylate; long- and short-chain alkyl acrylates such asmethyl acrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate,butyl acrylate, amyl acrylate, isobutyl acrylate, t-butyl acrylate,pentyl acrylate, isoamyl acrylate, hexyl acrylate, heptyl acrylate,octyl acrylate, isooctyl acrylate, 2-ethylhexyl acrylate, nonylacrylate, decyl acrylate, isodecyl acrylate, undecyl acrylate, dodecylacrylate, lauryl acrylate, octadecyl acrylate, and stearyl acrylate;aminoalkyl acrylates such as dimethylaminoethyl acrylate,diethylaminoethyl acrylate, and 7-amino-3,7-dimethyloctyl acrylate;alkoxyalkyl acrylates such as butoxyethyl acrylate, phenoxyethylacrylate (e.g., SR339, Sartomer Company, Inc.), and ethoxyethoxyethylacrylate; single and multi-ring cyclic aromatic or non-aromaticacrylates such as cyclohexyl acrylate, benzyl acrylate,dicyclopentadiene acrylate, dicyclopentanyl acrylate, tricyclodecanylacrylate, bomyl acrylate, isobornyl acrylate (e.g., SR423, SartomerCompany, Inc.), tetrahydrofiurfuryl acrylate (e.g., SR285, SartomerCompany, Inc.), caprolactone acrylate (e.g., SR495, Sartomer Company,Inc.), and acryloylmorpholine; alcohol-based acrylates such aspolyethylene glycol monoacrylate, polypropylene glycol monoacrylate,methoxyethylene glycol acrylate, methoxypolypropylene glycol acrylate,methoxypolyethylene glycol acrylate, ethoxydiethylene glycol acrylate,and various alkoxylated alkylphenol acrylates such as ethoxylated(4)nonylphenol acrylate (e.g., Photomer 4066, IGM Resins); acrylamides suchas diacetone acrylamide, isobutoxymethyl acrylamide,N,N′-dimethyl-aminopropyl acrylamide, N,N-dimethyl acrylamide, N,Ndiethyl acrylamide, and t-octyl acrylamide; vinylic compounds such asN-vinylpyrrolidone and N-vinylcaprolactam; and acid esters such asmaleic acid ester and fumaric acid ester. With respect to the long andshort chain alkyl acrylates listed above, a short chain alkyl acrylateis an alkyl group with 6 or less carbons and a long chain alkyl acrylateis alkyl group with 7 or more carbons.

Many suitable monomers are either commercially available or readilysynthesized using reaction schemes known in the art. For example, mostof the above-listed monofunctional monomers can be synthesized byreacting an appropriate alcohol or amide with an acrylic acid oracryloyl chloride.

Representative polyfunctional ethylenically unsaturated monomersinclude, without limitation, alkoxylated bisphenol A diacrylates, suchas ethoxylated bisphenol A diacrylate, with the degree of alkoxylationbeing 2 or greater. The monomer component of the secondary compositionmay include ethoxylated bisphenol A diacrylate with a degree ofethoxylation ranging from 2 to about 30 (e.g. SR349 and SR601 availablefrom Sartomer Company, Inc. West Chester, Pa. and Photomer 4025 andPhotomer 4028, available from IGM Resins), or propoxylated bisphenol Adiacrylate with the degree of propoxylation being 2 or greater; forexample, ranging from 2 to about 30; methylolpropane polyacrylates withand without alkoxylation such as ethoxylated trimethylolpropanetriacrylate with the degree of ethoxylation being 3 or greater; forexample, ranging from 3 to about 30 (e.g., Photomer 4149, IGM Resins,and SR499, Sartomer Company, Inc.); propoxylated-trimethylolpropanetriacrylate with the degree of propoxylation being 3 or greater; forexample, ranging from 3 to 30 (e.g., Photomer 4072, IGM Resins andSR492, Sartomer); ditrimethylolpropane tetraacrylate (e.g., Photomer4355, IGM Resins); alkoxylated glyceryl triacrylates such aspropoxylated glyceryl triacrylate with the degree of propoxylation being3 or greater (e.g., Photomer 4096, IGM Resins and SR9020, Sartomer);erythritol polyacrylates with and without alkoxylation, such aspentaerythritol tetraacrylate (e.g., SR295, available from SartomerCompany, Inc. (West Chester, Pa.)), ethoxylated pentaerythritoltetraacrylate (e.g., SR494, Sartomer Company, Inc.), anddipentaerythritol pentaacrylate (e.g., Photomer 4399, IGM Resins, andSR399, Sartomer Company, Inc.); isocyanurate polyacrylates formed byreacting an appropriate functional isocyanurate with an acrylic acid oracryloyl chloride, such as tris-(2-hydroxyethyl) isocyanuratetriacrylate (e.g., SR368, Sartomer Company, Inc.) andtris-(2-hydroxyethyl) isocyanurate diacrylate; alcohol polyacrylateswith and without alkoxylation such as tricyclodecane dimethanoldiacrylate (e.g., CD406, Sartomer Company, Inc.) and ethoxylatedpolyethylene glycol diacrylate with the degree of ethoxylation being 2or greater; for example, ranging from about 2 to 30; epoxy acrylatesformed by adding acrylate to bisphenol A diglycidylether and the like(e.g., Photomer 3016, IGM Resins); and single and multi-ring cyclicaromatic or non-aromatic polyacrylates such as dicyclopentadienediacrylate and dicyclopentane diacrylate.

In addition to functioning as a polymerizable moiety, monofunctionalmonomers may also be included in the curable secondary composition forother purposes. Monofunctional monomer components may, for example,influence the degree to which the cured product absorbs water, adheresto other coating materials, or behaves under stress.

The secondary composition may or may not include an oligomericcomponent. As indicated hereinabove, if present, oligomers may bepresent as a minor constituent in the secondary composition. One or moreoligomers may be present in the secondary composition. One class ofoligomers that may be included in the secondary composition isethylenically unsaturated oligomers. When employed, suitable oligomersmay be monofunctional oligomers, polyfunctional oligomers, or acombination of a monofunctional oligomer and a polyfunctional oligomer.If present, the oligomer component of the secondary composition mayinclude aliphatic and aromatic urethane (meth)acrylate oligomers, urea(meth)acrylate oligomers, polyester and polyether (meth)acrylateoligomers, acrylated acrylic oligomers, polybutadiene (meth)acrylateoligomers, polycarbonate (meth)acrylate oligomers, and melamine(meth)acrylate oligomers or combinations thereof. The secondarycomposition may be free of urethane groups, urethane acrylate compounds,urethane oligomers, or urethane acrylate oligomers.

The oligomeric component the secondary composition may include adifunctional oligomer. A difunctional oligomer may have a structureaccording to formula (I) below:F₁—R₁-[urethane-R₂-urethane]_(m)-R₁—F₁  (I)where F₁ may independently be a reactive functional group such asacrylate, methacrylate, acrylamide, N-vinyl amide, styrene, vinyl ether,vinyl ester, or other functional group known in the art; R₁ may include,independently, —C₂₋₁₂O—, —(C₂₋₄—O)_(n)—, —C₂₋₁₂O—(C₂₋₄—O)_(n)—,—C₂₋₁₂O—(CO—C₂₋₅O)_(n)—, or —C₂₋₁₂O—(CO—C₂₋₅NH)_(n)— where n is a wholenumber from 1 to 30, including, for example, from 1 to 10; R₂ may be apolyether, polyester, polycarbonate, polyamide, polyurethane, polyurea,or combination thereof; and m is a whole number from 1 to 10, including,for example, from 1 to 5. In the structure of formula (I), the urethanemoiety may be the residue formed from the reaction of a diisocyanatewith R₂ and/or R₁. The term “independently” is used herein to indicatethat each F₁ may differ from another F₁ and the same is true for eachR₁.

The oligomer component of the curable secondary composition may includea polyfunctional oligomer. The polyfunctional oligomer may have astructure according to formula (II), formula (III), or formula (IV) setforth below:multiurethane-(F₂—R₁—F₂)_(x)  (II)polyol-[(urethane-R₂-urethane)_(m)-R₁—F₂]_(x)  (III)multiurethane-(R₁—F₂)_(x)  (IV)where F₂ may independently represent from 1 to 3 functional groups suchas acrylate, methacrylate, acrylamide, N-vinyl amide, styrene, vinylether, vinyl ester, or other functional groups known in the art; R₁ caninclude —C₂₋₁₂O—, —(C₂₋₄—O)_(n)—, —C₂₋₁₂O—(C₂₋₄—O)_(n)—,—C₂₋₁₂O—(CO—C₂₋₅O)_(n)—, or —C₂₋₁₂O—(CO—C₂₋₅NH)_(n)— where n is a wholenumber from 1 to 10, including, for example, from 1 to 5; R₂ may bepolyether, polyester, polycarbonate, polyamide, polyurethane, polyureaor combinations thereof; x is a whole number from 1 to 10, including,for example, from 2 to 5; and m is a whole number from 1 to 10,including, for example, from 1 to 5. In the structure of formula (II),the multiurethane group may be the residue formed from reaction of amultiisocyanate with R₂. Similarly, the urethane group in the structureof formula (III) may be the reaction product formed following bonding ofa diisocyanate to R₂ and/or R₁.

Urethane oligomers may be prepared by reacting an aliphatic or aromaticdiisocyanate with a dihydric polyether or polyester, most typically apolyoxyalkylene glycol such as a polyethylene glycol. Moisture-resistantoligomers may be synthesized in an analogous manner, except that polarpolyethers or polyester glycols are avoided in favor of predominantlysaturated and predominantly nonpolar aliphatic diols. These diols mayinclude alkane or alkylene diols of from about 2-250 carbon atoms thatmay be substantially free of ether or ester groups.

Polyurea elements may be incorporated in oligomers prepared by thesemethods, for example, by substituting diamines or polyamines for diolsor polyols in the course of synthesis. The presence of minor proportionsof polyureas in the secondary coating composition is not considereddetrimental to coating performance, provided that the diamines orpolyamines employed in the synthesis are sufficiently non-polar andsaturated as to avoid compromising the moisture resistance of thesystem.

The secondary coating compositions may also contain a polymerizationinitiator to facilitate polymerization (curing) of the secondarycomposition after its application to a glass fiber or a glass fiberpreviously coated with a primary or other layer. Polymerizationinitiators suitable for use in the compositions of the present inventionmay include thermal initiators, chemical initiators, electron beaminitiators, microwave initiators, actinic-radiation initiators, andphotoinitiators. For many acrylate-based coating formulations,photoinitiators, such as the known ketonic photoinitiating and/orphosphine oxide additives, may be used. When used in the compositions ofthe present invention, the photoinitiator may be present in an amountsufficient to provide rapid ultraviolet curing. The photoinitiator maybe present in an amount ranging from about 0.5 wt % to about 10 wt %, orfrom about 1.5 wt % to about 7.5 wt %, or in an amount of about 3 wt %.

The amount of photoinitiator may be adjusted to promote radiation cureto provide reasonable cure speed without causing premature gelation ofthe coating composition. A desirable cure speed may be a speedsufficient to cause curing of the coating composition of greater thanabout 90%, or greater than 95%). As measured in a dose versus moduluscurve, a cure speed for coating thicknesses of about 75 μm may be, forexample, less than 1.0 J/cm² or less than 0.5 J/cm².

Suitable photoinitiators may include, without limitation,2,4,6-trimethylbenzoyl-diphenylphosphine oxide (e.g. Lucirin TPO);1-hydroxycyclohexylphenyl ketone (e.g. Irgacure 184 available fromBASF); (2,6-diethoxybenzoyl)-2,4,4-trimethylpentyl phosphine oxide (e.g.in commercial blends Irgacure 1800, 1850, and 1700, BASF);2,2-dimethoxyl-2-phenyl acetophenone (e.g., Irgacure, 651, BASF);bis(2,4,6-trimethylbenzoyl)phenyl phosphine oxide (e.g., Irgacure 819,BASF); (2,4,6-triiethylbenzoyl)diphenyl phosphine oxide (e.g., incommercial blend Darocur 4265, BASF);2-hydroxy-2-methyl-1-phenylpropane-1-one (e.g., in commercial blendDarocur 4265, BASF) and combinations thereof.

In addition to the above-described components, the secondary coatingcomposition of the present invention may optionally include an additiveor a combination of additives. Representative additives include, withoutlimitation, antioxidants, catalysts, lubricants, low molecular weightnon-crosslinking resins, adhesion promoters, and stabilizers. Additivesmay operate to control the polymerization process, thereby affecting thephysical properties (e.g., modulus, glass transition temperature) of thepolymerization product formed from the composition. Additives may affectthe integrity of the polymerization product of the composition (e.g.,protect against de-polymerization or oxidative degradation).

The secondary composition may include thiodiethylenebis(3,5-di-tert-butyl)-4-hydroxyhydrocinnamate (e.g., Irganox 1035,available from BASF) as an antioxidant. The secondary composition mayinclude an acrylated acid adhesion promoter (such as Ebecryl 170(available from UCB Radcure (Smyrna Ga.)). Other suitable materials foruse in secondary coating materials, as well as considerations related toselection of these materials, are well known in the art and aredescribed in U.S. Pat. Nos. 4,962,992 and 5,104,433, the disclosures ofwhich are hereby incorporated by reference.

Even with low oligomer content, the present secondary compositions mayresult in a secondary coating material having high tensile strength anda high modulus of elasticity (Young's modulus). The secondary coatingmay have a higher modulus of elasticity and higher glass transitiontemperature than the primary coating. As used herein, in situ modulus ofthe secondary coating is the modulus measured by the technique that isnow described.

Secondary In Situ Modulus

Secondary in situ modulus is measured using fiber tube-off samples. Toobtain a fiber tube-off sample, a 0.0055 inch Miller stripper is firstclamped down approximately 1 inch from the end of the coated fiber. Theone-inch region of fiber extending from the stripper is plunged into astream of liquid nitrogen and held for 3 seconds. The fiber is thenremoved from the stream of liquid nitrogen and quickly stripped. Thestripped end of the fiber is inspected to insure that the coating isremoved. If coating remains on the glass, the sample is prepared again.The result is a hollow tube of primary and secondary coatings. Thediameters of the glass, primary coating and secondary coating aremeasured from the end-face of the unstripped fiber.

To measure secondary in situ modulus, fiber tube-off samples can be runwith an instrument such as a Rheometrics DMTA IV instrument at a samplegauge length 11 mm. The width, thickness, and length of the sample aredetermined and provided as input to the operating software of theinstrument. The sample is mounted and run using a time sweep program atambient temperature (21° C.) using the following parameters:

-   -   Frequency: 1 Rad/sec    -   Strain: 0.3%    -   Total Time=120 sec.    -   Time Per Measurement=1 sec    -   Initial Static Force=15.0 [g]    -   Static>Dynamic Force by=10.0 [%]

Once completed, the last five E′ (storage modulus) data points areaveraged. Each sample is run three times (fresh sample for each run) fora total of fifteen data points. The averaged value of the three runs isreported as the secondary in situ modulus.

The tensile strength of the polymerization product of the secondarycomposition of the present disclosure, when prepared in the form ofcured rods, may be at least 50 MPa. When measured on cured coating rodsat room temperature (−21° C.), the modulus of elasticity of the curedproduct of the secondary composition may be in the range from about 1400MPa to about 2200 MPa, or in the range from about 1700 MPa to about 2100MPa, or in the range from about 1600 MPa to about 3000 MPa. The in situmodulus of elasticity of the secondary coating may be 1200 MPa orgreater, or 1500 MPa or greater, or 1800 MPa or greater, or 2100 MPa orgreater, or 2400 MPa or greater, or 2700 MPa or greater.

Young's Modulus, Tensile Strength and % Elongation at Break

Coating compositions are prepared in the form of rod samples for tensiletesting. Rods are prepared by injecting the curable compositions intoTeflon® tubing having an inner diameter of about 0.025″. The rod samplesare cured using a Fusion D bulb at a dose of about 2.4 J/cm² (measuredover a wavelength range of 225-424 nm by a Light Bug model IL390 fromInternational Light). After curing, the Teflon® tubing is stripped awayto provide rod samples of the coating composition. The cured rods areallowed to condition overnight at 23° C. and 50% relative humidity.Properties such as Young's modulus, tensile strength, and % elongationat break are measured using a tensile testing instrument (e.g., aSintech MTS Tensile Tester, or an Instron Universal Material TestSystem) on defect-free rod samples with a gauge length of 51 mm, and atest speed of 250 mm/min. The properties are determined as an average ofat least five samples, with defective samples being excluded from theaverage.

High modulus secondary coatings may offer better protection of the fiberagainst mechanical damage and better microbend performance. However,high speed processing of high modulus secondary coatings in the drawtower may be a challenge because of an increased tendency to of the drawprocess to generate defects such as flat spots and wind induced pointdefects (WIPD) in the secondary coating that ultimately compromise fiberperformance.

During the development of oligomer-free coatings urethane-oligomer-freecoatings and urethane-acrylate-oligomer-free coatings, it was found thatremoval of the oligomer from the formulation, without modifying othercomponents, may result in a secondary coating with a modulus of higherthan 2000 MPa. Such secondary coatings that may be difficult to processat high speeds in the draw tower. Accordingly, it may be desirable tocompensate for the effect of removing the oligomer by formulating thesecondary composition to include monomers with long flexible (e.g.ethoxylated) chains between functional groups. Long flexible chains mayincrease the distance between crosslinks, may decrease the crosslinkdensity and may ultimately lower the modulus of the cured secondarycoating. A potential drawback of such monomers is that they may have alower glass transition temperature (Tg) and may tend to decrease the Tgof the cured secondary coating. Secondary coatings with low Tg may notbe desirable because a low Tg may result in a material that is too softat the time of application and may lead to defects during processing athigh speed. Higher Tg secondary coatings may be harder at roomtemperature and may provide better mechanical protection to the opticalfiber. If the Tg is too high, however, the coating may be sufficientlystiff to make the fiber more prone to defects during processing.

The secondary coating of the present disclosure may be designed toachieve a secondary coating with moderate Tg that imparts adequatemechanical protection and bend insensitivity to the optical fiber whilestill allowing the fiber to be processed defect-free in high speed drawtowers. Tg can be measured using the technique that is now described.

Glass transition temperatures are measured using samples in the form ofcured films (primary coating) or rods (secondary coating) formed fromthe coating composition. Glass transition temperatures are measured bydetermining the peak of the tan δ curves obtained from an instrumentsuch as a Rheometrics DMTA IV in tension. The width, thickness, andlength of the sample are input to the “Sample Geometry” section of theprogram. The sample is mounted and then cooled to approximately −85° C.Once stable, a temperature ramp is run using the following parameters:

-   -   Frequency: 1 Hz    -   Strain: 0.3%    -   Heating Rate: 2° C./min.    -   Final Temperature: 150° C.    -   Initial Static Force=20.0 [g]    -   Static>Dynamic Force by=10.0 [%]

Tg is defined as the maximum of the tan δ peak, where the tan δ peak isdefined as:tan δ=E″/E′where E″ is the loss modulus, which is proportional to the loss ofenergy as heat in a cycle of deformation and E′ is the storage orelastic modulus, which is proportional to the energy stored in a cycleof deformation.

The Tg of cured rods prepared from the cured product of the secondarycoating composition may be at least about 50° C. The glass transitiontemperature of the secondary coating may be at least 50° C., or at least55° C., or at least 60° C., or between 55° C. and 65° C.

The secondary composition may be devoid of an oligomeric component, aurethane oligomeric component, or a urethane-acrylate oligomericcomponent, and the monomeric component may include ethoxylated(4)bisphenol-A diacrylate monomer, ethoxylated(30) bisphenol-A diacrylatemonomer, and epoxy diacrylate monomer. The ethoxylated(4) bisphenol-Adiacrylate monomer may be present in an amount ranging from about 50 wt% to about 90 wt %, or from about 60 wt % to about 80 wt %, or and fromabout 70 wt % to about 75 wt %. The ethoxylated(30) bisphenol-Adiacrylate monomer may be present in an amount ranging from about 5 wt %to about 20 wt %, or from about 7 wt % to about 15 wt %, or from about 8wt % to about 12 wt %. The epoxy diacrylate monomer may be present in anamount of ranging from about 5 wt % to about 25 wt %, or from about 10wt % to about 20 wt %, or from about 12 wt % to about 18 wt %.

The secondary composition may be devoid of an oligomeric component, aurethane oligomeric component, or a urethane-acrylate oligomericcomponent, and the monomeric component may include ethoxylated(4)bisphenol-A diacrylate monomer, ethoxylated(10) bisphenol-A diacrylatemonomer, and epoxy diacrylate monomer. The ethoxylated(4) bisphenol-Adiacrylate monomer may be present in an amount ranging from about 30 wt% to about 80 wt %, or from about 40 wt % to about 70 wt %, or fromabout 50 wt % to about 60 wt %. The ethoxylated(10) bisphenol-Adiacrylate monomer may be present in an amount ranging from about 10 wt% to about 50 wt %, or from about 20 wt % to about 40 wt %, or fromabout 25 wt % to about 35 wt %. The epoxy diacrylate monomer may bepresent in an amount ranging from about 5 wt % to about 25 wt %, or fromabout 10 wt % to about 20 wt %, or from about 12 wt % to about 18 wt %.

The secondary composition may be devoid of an oligomeric component, aurethane oligomeric component, or a urethane-acrylate oligomericcomponent, and the monomeric component may include ethoxylated(4)bisphenol-A diacrylate monomer, ethoxylated(10) bisphenol-A diacrylatemonomer, ethoxylated(30) bisphenol-A diacrylate monomer, and epoxydiacrylate monomer. The ethoxylated(4) bisphenol-A diacrylate monomermay be present in an amount ranging from about 40 wt % to about 80 wt %,or from about 60 wt % to about 70 wt %. The ethoxylated(10) bisphenol-Adiacrylate monomer may be present in an amount ranging from about 1 wt %to about 30 wt %, or from about 5 wt % to about 10 wt %. Theethoxylated(30) bisphenol-A diacrylate monomer may be present in anamount ranging from about 5 wt % to about 20 wt %, or in an amount ofabout 10 wt %. The epoxy diacrylate monomer may be present in an amountranging from about 5 wt % to about 25 wt %, or in an amount of about 15wt %.

The secondary composition may be devoid of an oligomeric component, aurethane oligomeric component, or a urethane-acrylate oligomericcomponent, and the monomeric component may include ethoxylated (10)bisphenol A diacrylate monomer, tripropylene glycol diacrylate monomer,ethoxylated (4) bisphenol A diacrylate monomer, and epoxy diacrylatemonomer. The ethoxylated (10) bisphenol A diacrylate monomer may bepresent in an amount ranging from about 10 wt % to about 50 wt %. Thetripropylene glycol diacrylate monomer may be present in an amount fromabout 5 wt % to about 40 wt %. The ethoxylated (4) bisphenol Adiacrylate monomer may be present in an amount from about 10 wt % toabout 55 wt %. The epoxy diacrylate monomer may be present in an amountup to about 15 wt %.

The secondary composition may comprise from about 40 wt % to 80 wt % ofethoxylated (4) bisphenol A monomer, from about 0 wt % to about 30% ofethoxylkated (10) bisphenol A monomer, from about 0 wt % to about 25% wt% of ethoxylated (30) bisphenol A monomer, from about 5 wt % to 18 wt %of epoxy acrylate, from about 0 wt % to 10 wt % of tricyclodecanedimethanoldiacrylate monomer, from about 0.1 wt % to 40% of one or morephotoinitiators, from about 0 pph to 5 pph by weight of slip additive;and from 0 pph to about 5 pph by weight of an antioxidant. The secondarycomposition may further comprise 3% or less oligomer, or 1% or lessoligomer, or may be devoid of oligomer. The epoxy acrylate may be anepoxy acrylate monomer. The epoxy acrylate may be bisphenol A epoxydiacrylate. The epoxy acrylate may be an unmodified epoxy acrylate, forexample an epoxy acrylate which is not modified with fatty acid, amine,acid, or aromatic functionality. Such compositions may have a viscosityat 45° C. of at least about 3 Poise and when cured, may exhibit aYoung's modulus of from about 1400 MPa to about 2100 MPa. Thecompositions may exhibit a glass transition temperature of at leastabout 55° C. The monomeric component may include an alkoxylatedbisphenol A diacrylate monomer having at least 10 alkoxy groups.

The primary and secondary curable compositions may be applied to theglass portion of the coated fiber after it has been drawn from thepreform. The primary and secondary compositions may be appliedimmediately after cooling. The curable compositions may then be cured toproduce the coated optical fiber. The method of curing may be thermal,chemical, or radiation induced, such as by exposing the applied curablecomposition on the glass fiber to ultraviolet light, actinic radiation,microwave radiation, or an electron beam, depending upon the nature ofthe coating composition(s) and polymerization initiator being employed.It may be advantageous to apply both a primary curable composition and asecondary curable composition in sequence following the draw process.Methods of applying dual layers of curable compositions to a movingglass fiber are disclosed in U.S. Pat. Nos. 4,474,830 and 4,585,165, thedisclosures of which are hereby incorporated by reference herein. Theprimary curable composition may alternatively be applied and cured toform the primary coating material before applying and curing thesecondary curable composition to form the secondary coating.

EXAMPLES

Various exemplary coated fibers in accordance with the presentdisclosure are now described and modeled to illustrate one or moreadvantageous features disclosed herein.

The coated fibers modeled for these examples included a glass fiber witha diameter of 125 μm. The core of the glass fiber had a radius rangingbetween 4 to 10 μm and may be made by modifying silica with GeO₂ toincrease the index of the core relative to the cladding. The claddingsurrounded the core, extended to a radius of 62.5 μm and included aninner cladding region and an outer cladding region where the innercladding region had a lower index than the outer cladding. The lowerindex of the inner cladding region relative to the outer cladding may beachieved by doping the inner cladding with the downdopant fluorine.Alternatively, the higher index of the outer cladding region relative tothe inner cladding region may be achieved by doping the outer claddingwith updopants such as chlorine, germania, alumina, titania, siliconoxynitride, phosphorus, etc. Exemplary refractive index profiles will bediscussed more fully hereinbelow.

Representative curable compositions A-H for the primary coating areshown in Table I below.

TABLE I Illustrative Primary Coating Compositions Component A B C D E FG H Photomer 4066 (wt %) 41.5 0 61.5 41.5 46.5 46.5 45.5 47 Photomer4960 (wt %) 0 41.5 0 0 0 0 0 0 BR3741 (wt %) 55 55 35 55 50 50 50 50N-vinyl caprolactam (wt %) 2 2 2 2 2 2 2 1.5 TPO (wt %) 1.5 1.5 1.5 1.51.5 1.5 2.5 1.5 (3-acryloxypropyl) 1 1 1 1 1 0.8 0.8 0.8trimethoxysilane (pph) Irganox 1035 (pph) 1 1 1 1 1 1 1 1Pentaerythritol 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03mercaptopropionate (pph) Uvitex OB (pph) 0.05 0 0 0 0 0 0 0

Photomer 4066 is an ethoxylated nonyl phenol acrylate available from IGMResins. Photomer 4960 is a propoxylated nonyl phenol acrylate availablefrom IGM Resins. BR3741 is an aliphatic polyether urethane acrylateoligomer available from Dymax Oligomers and Coatings. N-vinylcaprolactam is available from ISP Technologies, Inc. TPO((2,4,6-trimethylbenzoyl)diphenyl phosphine oxide) is a photoinitiatoravailable from BASF. (3-acryloxypropyl) trimethoxysilane is an adhesionpromoter available from Gelest. IRGANOX 1035 ((thiodiethylenebis[3-(3,5-di-tert-butyl)-4-hydroxyphenyl) propionate]) is anantioxidant available from BASF. Pentaerythritol mercaptopropionate isan adhesion promoter stabilizer available from Aldrich. UVITEX OB(C₂₆H₂₆N₂O₂S, CAS No. 7128-64-5) is an optical brightener available fromBASF.

To prepare the primary composition, the oligomer and monomer(s) may beblended together for at least 10 minutes at 60° C. Photoinitiator(s) andadditives may then be added, and blending may be continued for one hour.Finally, the adhesion promoter may be added, and blending may becontinued for 30 minutes. The resulting solution may then be applied tothe glass portion of the fiber and UV-cured to form a primary coating.

Representative curable compositions J-L for the secondary coating areshown in Table II below.

TABLE II Illustrative Secondary Coating Compositions Component J K LSR601/Photomer4028 (wt %) 72 52 72 CD9038 (wt %) 10 0 10 Photomer3016(wt %) 15 15 15 SR602 (wt %) 30 wt % 0 30 0 Irgacure 184 (wt %) 1.5 1.51.5 TPO (wt %) 1.5 1.5 1.5 DC190 Fluid slip additive (pph) 0 0 1 Irganox1035 (pph) 0.5 1 1

SR601/Photomer 4028 is an ethoxylated (4)bisphenol A diacrylate monomeravailable from Sartomer or IGM Resins. CD9038 is an ethoxylated(30)bisphenol A diacrylate monomer available from Sartomer. Photomer3016 is an epoxy diacrylate monomer available from IGM Resins. SR602 isan ethoxylated (10)bisphenol A diacrylate monomer available fromSartomer. IRGACURE 184 (1-hydroxycyclohexylphenyl ketone) is aphotoinitiator available from BASF. TPO((2,4,6-trimethylbenzoyl)-diphenyl phosphine oxide) is a photoinitiatoravailable from BASF. DC190 is a fluid slip additive available from DowCorning. IRGANOX 1035 (thiodiethylenebis(3,5-di-tert-butyl)-4-hydroxyhydrocinnamate) is an antioxidantavailable from BASF.

Secondary compositions may be prepared with the listed components usingcommercial blending equipment. The monomer components may be weighed,introduced into a heated kettle, and blended together at a temperaturewithin the range of from about 50° C. to 65° C. Blending may then becontinued until a homogenous mixture is obtained. Next, thephotoinitiator may be weighed and introduced into the homogeneoussolution while blending. Finally, the remaining components may beweighed and introduced into the solution while blending. Blending may becontinued until a homogeneous solution is again obtained. Thehomogeneous solution may then be applied to the primary coating orprimary composition of the fiber and cured with UV radiation to form asecondary coating.

Coated fibers with a 125 μm-diameter core-cladding glass region andprimary and secondary coatings having properties consistent with thecoatings achievable by curing the primary and secondary compositionslisted in Tables I and II were modeled. The coated fiber characteristicsupon which the models were based are now described for five exemplarycoated fibers. The exemplary coated fibers will be referred to by samplenumbers 1, 2, 3, 4, and 5.

The refractive index profile, expressed in Δ% relative to pure silicaglass, for exemplary coated fiber 1 is shown in FIG. 4. Coated fiber 1included a core with an outer radius r₁ of 6 μm and index profile Δ₁based on an α-profile with α=2, with maximum core index Δ_(1MAX) of0.41%. An inner cladding of silica surrounded the core and extended toan outer radius r₃ of 30 μm. An outer cladding surrounded the innercladding, extended to a radius r₄ of 62.5 μm and had index Δ₄ of about0.05%.

The refractive index profile, expressed in Δ % relative to pure silicaglass, for exemplary coated fiber 2 is shown in FIG. 5. Coated fiber 2included a core with an outer radius r₁ of 5.73 μm and index profile Δ₁based on an α-profile with α=2, with maximum core index Δ_(1MAX) of0.385%. A first inner cladding of silica surrounded the core andextended to an outer radius r₂ of 6.88 μm. A second inner claddingsurrounded the first inner cladding and extended to an outer radius r₃of 17.2 μm. The second inner cladding included a fluorine-dopedtriangular trench region that provided a linear decrease in index fromΔ₃=0% at radius r₂ to Δ₃=−0.2% at a radius r₃. An outer cladding ofsilica surrounded the second inner cladding and extended to a radius r₄of 62.5 μm.

In the modelling, exemplary coated fibers 1 and 2 were each treated ashaving a primary coating with an outer diameter of 165 μm and an in situmodulus of less than 0.50 MPa, and a secondary coating with an outerdiameter of 200 μm and an in situ modulus of greater than 1600 MPa. Theprimary and secondary coating compositions listed in Tables 1 and 2 areexpected to yield cured primary and secondary coatings having thesecharacteristics.

The modelled characteristics of exemplary coated fibers 1 and 2 arepresented in Table III. Modelled performance data included cutoffwavelength (LP11 mode and cable), mode field diameter (at 1310 nm and1550 nm), zero dispersion wavelength, dispersion and dispersion slope at1310 nm and 1550 nm, macrobending losses at 1550 nm (based on themandrel wrap test using mandrels with diameters of 10 mm, 20 mm, and 30mm), and microbending losses at 1550 nm (based on the pin array andlateral load tests). The modelling data show that the exemplary coatedfibers 1 and 2 are low-diameter fibers that exhibit (1) a mode fielddiameter compatible with efficient splicing and connection to standardsingle-mode fibers and (2) low bending losses.

TABLE III Fiber Performance Data Parameter Coated Fiber 1 Coated Fiber 21310 nm MFD (μm) 9.2 9.2 1550 nm MFD (μm) 10.51 10.43 Zero DispersionWavelength (nm) 1319 1320 Dispersion at 1310 nm (ps/nm/km) −0.801 −0.909Dispersion Slope at 1310 nm 0.089 0.0909 (ps/nm²/km) Dispersion at 1550nm (ps/nm/km) 17.27 17.65 Dispersion Slope at 1550 nm 0.06 0.0626(ps/nm²/km) Cable Cutoff (22-meter) (nm) 1209 1229 Bend Loss at 1550nm-10 mm Diameter 1.13 1.52 Mandrel (dB/turn) Bend Loss at 1550 nm-15 mmDiameter 0.279 0.337 Mandrel (dB/turn) Bend Loss at 1550 nm-20 mmDiameter 0.068 0.074 Mandrel (dB/turn) Bend Loss at 1550 nm-30 mmDiameter 0.006 0.003 Mandrel (dB/turn) Bend Loss at 1550 nm-Pin Array(dB) 21.73 9.89 Bend Loss at 1550 nm-Lateral 0.229 0.223 Load (dB)

The refractive index profile, expressed in Δ % relative to pure silicaglass, for exemplary coated fiber 3 is shown in FIG. 6. The refractiveindex profile of each of the coated fibers 3, 4, and 5 was similar inprofile shape and the profiles are described in Table IV. Each ofexemplary coated fibers 3, 4, and 5 included a high index core thatextended to an outer radius r₁ of about 4.5 μm, a first inner claddingregion extending between radius r₁ and an outer radius r₂ of about 7 μm,a second inner cladding region between radius r₂ and an outer radius r₃of about 18 μm, and an outer cladding region extending between radius r₃and an outer radius r₄ of 62.5 μm. The refractive index of the secondinner cladding region continuously decreased with increasing radialposition as shown to form a triangular trench in the index profile ofexemplary coated fibers 3, 4, and 5.

In the modelling, exemplary coated fibers 3, 4, and 5 were each treatedas having a primary coating with an outer diameter of 165 μm and an insitu modulus of less than 0.5 MPa, and a secondary coating with an outerdiameter of 200 μm and an in situ modulus of greater than 1600 MPa. Theprimary and secondary coating compositions listed in Tables 1 and 2 areexpected to yield cured primary and secondary coatings having thesecharacteristics.

The modelled characteristics of exemplary coated fibers 3, 4, and 5 arepresented in Table IV. Table IV lists numerical values for therefractive index and radial positions of the core region (region 1),first inner cladding region (region 2), second inner cladding region(region 3), and outer cladding region (region 4). Modelled performancedata included cutoff wavelength (core and cable), mode field diameter(for both core and fiber at 1310 nm), zero dispersion wavelength, coreMAC number (ratio of mode field diameter at 1310 nm to the cable cutoffwavelength), and macrobending losses at 1550 nm (based on the mandrelwrap test using mandrels with diameters of 10 mm, 20 mm, and 30 mm).

The model indicates that each of exemplary coated fibers 3, 4, and 5exhibited a MFD at 1310 nm greater than 9 μm, macrobend losses at awavelength of 1550 nm of less than 0.35 dB/turn for a 10 mm diametermandrel, less than 0.09 dB/turn for a 15 mm diameter mandrel, less than0.025 dB/turn for a 20 mm mandrel and less than 0.004 dB/turn for a 30mm diameter mandrel. Each of exemplary coated fibers 3, 4, and 5exhibited a cable cutoff of less than 1260 nm and a zero dispersionwavelength between 1.3 μm and 1.324 μm. The modelling results show thatexemplary coated fibers 3, 4, and 5 are low-diameter fibers that exhibitboth (1) a mode field diameter compatible with efficient splicing andconnection to standard single-mode fibers and (2) low bending losses.

TABLE IV Fiber Performance Data Coated Coated Coated Parameter Fiber 3Fiber 4 Fiber 5 Δ₁, max (%) 0.345 0.34 0.335 r₁ (μm) 4.55 4.58 4.6 Δ₂(%) 0 0 0 r₂ (μm) 7.17 7.2 7.25 Δ₃, min (%) −0.435 −0.435 −0.435 r₃ (μm)17.7 17.8 17.9 Δ₄ (%) 0 0 0 Moat Volume (% μm²) 62.93 63.65 64.36 MFD at1310 nm (μm) 9.12 9.18 9.24 Zero Dispersion Wavelength (μm) 1.321 1.3211.32 Cable Cutoff (22-meter) (nm) 1226 1226 1226 Bend Loss at 1550 nm-10mm 0.27 0.276 0.31 Diameter Mandrel (dB/turn) Bend Loss at 1550 nm-15 mm0.07 0.074 0.08 Diameter Mandrel (dB/turn) Bend Loss at 1550 nm-20 mm0.017 0.019 0.02 Diameter Mandrel (dB/turn) Bend Loss at 1550 nm-30 mm0.003 0.003 0.0037 Diameter Mandrel (dB/turn)

Unless otherwise expressly stated, it is in no way intended that anymethod set forth herein be construed as requiring that its steps beperformed in a specific order. Accordingly, where a method claim doesnot actually recite an order to be followed by its steps or it is nototherwise specifically stated in the claims or descriptions that thesteps are to be limited to a specific order, it is no way intended thatany particular order be inferred.

It will be apparent to those skilled in the art that variousmodifications and variations can be made without departing from thespirit or scope of the invention. Since modifications combinations,sub-combinations and variations of the disclosed embodimentsincorporating the spirit and substance of the invention may occur topersons skilled in the art, the invention should be construed to includeeverything within the scope of the appended claims and theirequivalents.

What is claimed is:
 1. A single-mode optical fiber comprising: a core,said core having an outer radius r₁; a cladding surrounding said core,said cladding having an outer radius r₄, said cladding including adepressed index region, said depressed index region having a triangularrelative refractive index profile with a moat volume with a magnitudeless than 75% μm²; a primary coating surrounding said cladding, saidprimary coating having an outer radius r₅, said primary coating havingan in situ modulus of 0.50 MPa or less; and a secondary coatingsurrounding said primary coating, said secondary coating having an outerradius r₆, said secondary coating having an in situ modulus of 1500 MPaor greater; wherein said outer radius r₆ is 110 μm or less and saidsingle-mode optical fiber has a mode field diameter 9.1 μm or greater at1310 nm, a cable cutoff wavelength of 1260 nm or less, a zero dispersionwavelength λ₀ in the range 1300 nm≤λ₀≤1324 nm, a MAC number greater than7.4 at 1310 nm, and said single-mode optical fiber exhibits a bend lossat a wavelength of 1550 nm, when turned about a mandrel having adiameter of 15 mm, of less than 0.5 dB/turn.
 2. The fiber of claim 1,wherein said cladding includes an inner cladding region having an outerradius r₃ and an outer cladding region surrounding said inner claddingregion and having said outer radius r₄, said inner cladding regionhaving a refractive index Δ₃ with a minimum value Δ_(3MIN), said outercladding region having a refractive index Δ₄ with a minimum valueΔ_(4MIN)>Δ_(3MIN).
 3. The fiber of claim 2, wherein said inner claddingregion is directly adjacent to said core and said refractive index Δ₃ issubstantially constant between said outer radius r₁ and said outerradius r₃.
 4. The fiber of claim 2, wherein said inner cladding regionis directly adjacent to said core, said refractive index Δ₃ decreasinglinearly between said outer radius r₁ and said outer radius r₃.
 5. Thefiber of claim 4, wherein said outer cladding region is directlyadjacent said inner cladding region and said refractive index Δ₄ issubstantially constant between said outer radius r₃ and said outerradius r₄.
 6. The fiber of claim 1, wherein said cladding includes afirst inner cladding region having an outer radius r₂, a second innercladding region surrounding said first inner cladding region and havingan outer radius r₃, and an outer cladding region surrounding said secondinner cladding region and having said outer radius r₄, said first innercladding region having a refractive index Δ₂ with a minimum valueΔ_(2MIN), said second inner cladding region having a refractive index Δ₃with a minimum value Δ_(3MIN)<Δ_(2MIN), said outer cladding regionhaving a refractive index Δ₄ with a minimum value Δ_(4MIN)>Δ_(3MIN). 7.The fiber of claim 6, wherein second inner cladding region is directlyadjacent said first inner cladding region and said refractive index Δ₃is substantially constant between said outer radius r₂ and said outerradius r₃.
 8. The fiber of claim 6, wherein second inner cladding regionis directly adjacent said first inner cladding region and saidrefractive index Δ₃ decreases linearly between said outer radius r₂ andsaid outer radius r₃.
 9. The single-mode optical fiber of claim 1,wherein said outer radius r₄ is at least 60 μm.
 10. The single-modeoptical fiber of claim 1, wherein said outer radius r₅ is 85 μm or less.11. The single-mode optical fiber of claim 1, wherein said outer radiusr₆ is 100 μm or less.
 12. The single-mode optical fiber of claim 1,wherein said single-mode optical fiber exhibits a bend loss at awavelength of 1550 nm, when turned about a mandrel having a diameter of20 mm, of less than 0.25 dB/turn.
 13. The single-mode optical fiber ofclaim 1, wherein said primary coating has an in situ modulus of 0.20 MPaor less.
 14. The single-mode optical fiber of claim 1, wherein saidsecondary coating has an in situ modulus of 1800 MPa or greater.
 15. Thefiber of claim 1, wherein said primary coating has a glass transitiontemperature of less than −15° C.
 16. The fiber of claim 1, wherein saidprimary coating has a glass transition temperature higher than −50° C.17. The fiber of claim 1, wherein said primary coating has a glasstransition temperature higher than −60° C.
 18. The fiber of claim 1,wherein said primary coating is the cured product of a primarycomposition that includes a urethane acrylate oligomer and a monomerselected from the group consisting of (meth)acrylates, N-vinyl amides,and epoxy acrylates.
 19. The fiber of claim 18, wherein said primarycomposition comprises one or more monofunctional (meth)acrylate monomersin an amount of from about 5 to 95% by weight; an N-vinyl amide monomerin an amount of from about 0.1 to 40% by weight; and one or moredifunctional urethane acrylate oligomers which comprise a polyol and anisocyanate, said oligomer present in an amount of from about 5 to 95% byweight, wherein the polyol in said oligomer is a polypropylene glycoland the isocyanate in said oligomer is an aliphatic diisocyanate. 20.The fiber of claim 18, wherein said primary composition comprises one ormore monofunctional (meth)acrylate monomers in an amount of from about40 to 65% by weight; an N-vinyl amide monomer in an amount of from about2 to 10% by weight; and one or more polypropylene glycol-baseddifunctional urethane acrylate oligomers in an amount of from about 35to 60% by weight.
 21. The fiber of claim 1, wherein said secondarycoating is the cured product of a secondary composition that lacksurethane-containing oligomers.
 22. The fiber of claim 21, wherein saidsecondary composition comprises a monomer selected from the groupconsisting of alkoxylated bisphenol-A diacrylate, alkylene glycolacrylate, and epoxy diacrylate.
 23. The fiber of claim 22, wherein saidsecondary composition comprises: about 40 to 80% by weight ofethoxylated (4) bisphenol A monomer; from about 0 to about 30% by weightof ethoxylated (10) bisphenol A monomer; from about 0 to about 25% byweight of ethoxylated (30) bisphenol A monomer; and from about 5 to 18%by weight of epoxy acrylate.
 24. The single-mode optical fiber of claim1, wherein said mode field diameter is 9.2 μm or greater at 1310 nm. 25.The single-mode optical fiber of claim 1, wherein said single-modeoptical fiber exhibits a bend loss at a wavelength of 1550 nm, whenturned about a mandrel having a diameter of 20 mm, of less than 0.25dB/turn.
 26. The single-mode optical fiber of claim 1, wherein saidsingle-mode optical fiber exhibits a bend loss at a wavelength of 1550nm, when turned about a mandrel having a diameter of 15 mm, of less than0.25 dB/turn.
 27. The single-mode optical fiber of claim 1, wherein saidmode field diameter is 9.2 μm or greater at 1310 nm.
 28. The single-modeoptical fiber of claim 1, wherein said depressed index region of saidcladding has a moat volume of at least 30% μm².